Define Nitrogen Estimation - Kjeldahl's Method?

Nitrogen in the protein is estimated by a method referred to as Kjeldahl's method. Although over a period of time many other methods have emerged for determination of organic nitrogen, this method still remains an old favourite because it is reliable and has very well standardized procedures. The method involves the digestion of the weighed sample with concentrated sulphuric acid, selenium oxide, potassuim sulphate and copper sulphate in a long necked special digestion flask known as kjeldahl's flask. Potassium sulphate increases the boiling point of the mixture. Since the digestion results in fuming of the sulphuric acid, the procedure has to be carried out in a fume hood and the procedure takes several hours. The sample is digested until the solution in the digestion flask becomes clear after which it is transferred to an appropriate volumetric flask (usually 50 or 100 ml flask) and the volume is made up to the mark with distilled water. Aliquots of this sample are used for distillation in the kjeldahl's apparatus and nitrogen in the protein is estimated. Most proteins contains 16% nitrogen. Thus, percent nitrogen content is multiplied by 6.25 to get crude protein content.

The experiment involves decomposition of ammonium sulphate by a fixed volume of alkali (NaOH). Ammonium sulphate on decomposition releases ammonia which is collected in a flask of boric acid solution Tashiro's indicator. With boric acid ammonia forms a complex 'ammoium borate'. As ammonia is distilled into the boric acid, the Tashiro indicator slowly changes colour from pink to green. This is because the colour of the indicator is pH dependent - it is pink in acidic medium and green in alkaline medium. When the green coloured solution containing the boric-acid-ammonia complex is retitrated against H₂SO₄, the solution slowly becomes acidic again and the indicator becomes pink. This point when colour changes from green to pink is taken as the end point.